1. Field of the Invention
The present invention relates to a current limiting device for vehicle suitable for a vehicle provided with a motor generator not incorporating a high-capacity smoothing capacitor and a current sensor.
2. Background Art
JP-A-2006-25488 discloses that drive torque and generated output of a motor generator in a practical rotation region are better with a rotary electric machine using a coil wiring having a larger wire diameter and a smaller winding number, that is, a low-impedance coil wiring. Also, an increase of a voltage at a drive power supply of the motor allows a larger amount of current to flow, which increases drive torque. Hence, there can be achieved an advantage that the engine can start promptly and drive torque in a high rotation region is enhanced. By combining a high-voltage source and a motor generator using a low-impedance coil wiring, advantages of the both components can be utilized effectively without impairing each other.
Meanwhile, JP-A-2007-159353 discloses a method of achieving a simpler configuration and a size reduction by not only omitting a high-capacity smoothing capacitor and a current sensor by controlling a switching element in an inverter of a motor generator to switch ON and OFF using a rectangular wave drive method but also combining the inverter and a rotary electric machine into one unit. Besides a high-voltage source and a low-impedance coil wiring, by making the configuration of the motor generator simpler, a loss caused by wiring resistance between the inverter and the rotary electric machine can be lessened. It thus becomes possible to construct a more efficient system.
When the motor generator is motor-driven using a high-voltage source, however, a current equal to or exceeding a rated current of the coil wiring or a rated current of the switching element in the inverter of the motor generator flows in a very low rotation region. This raises a problem that the motor generator is broken.
In addition, as has been described above, the motor generator of a simpler configuration has no current sensor. Moreover, by combining the inverter and the rotary electric machine into one unit, these components are directly connected to the engine. Because a mounting space is strictly limited, there is no room to provide an additional current sensor on the motor generator. This raises a problem that neither can a bus current be detected nor can a current be limited.
Further, when the current limiting device is installed in the vicinity of the high-voltage source, for example, in a luggage room, the wiring becomes longer and surge energy caused by wiring inductance consequently increases. Accordingly, there is a need to increase a capacity of a surge absorbing capacitor of the motor generator, and this need raises a problem that the size is increased.